Claremont Chert

The crest of the north Oakland hills is commonly held up by this well-layered chert, mapped as part of the Claremont Formation. It’s of Miocene age, maybe around 15 million years old, and formed in quiet offshore waters not far from here. That is, there were no hills and no Bay here. Mount Tam, in the background, didn’t exist. The whole Coast Range didn’t exist, maybe a few island chains. The Sierra Nevada was a low, ancient range of hills. Since then the crust has been shredded and pressed, tilting these chert beds upright and even over.

This exposure is on Grizzly Peak Boulevard at the top of Marlborough Terrace. More of it crops out along Grizzly Peak and Skyline boulevards for several kilometers southward. Just past the road is the top of Claremont Canyon, most of which is within the Oakland city limits.